1 boat ultimately something to chew over (4)
I believe the answer is:
duct
'1' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'boat ultimately something to chew over' is the wordplay.
'ultimately' says to take the final letters.
'something to chew' becomes 'duc' (I can't explain this - if you can you should believe this answer much more).
'over' says to put letters next to each other.
The last letter of 'boat' is 't'.
't' put after 'duc' is 'DUCT'.
Can you help me to learn more?
(Other definitions for duct that I've seen before include "Pipe for cables or air" , "Pipe, tube" , "Tube for conveing cable, liquid etc" , "Tube for cables, air, etc" , "Channel" .)